In science’s struggle to keep up with life on the streets, smoking cannabis for medical purposes stands as Exhibit A.

Medical use of cannabis has taken on momentum of its own, surging ahead of scientists’ ability to measure the drug’s benefits. The pace has been a little too quick for some, who see medicinal joints as a punch line, a ruse to free up access to a recreational drug.

But while the medical marijuana movement has been generating political news, some researchers have been quietly ...